They force you to grip hard, which really teaches you to engage your forearm muscles to contract to maximum capacity. This leads to neuromuscular activation of your shoulder muscles (lats, pecs, teres major, rotator cuff muscles), and therefore leads to greater tension and force output/transfer. Basically, you get stronger. It is also can increase the effect of shoulder stability exercises for the same reason. I'm not saying it will have a huge effect, but it will encourage strength gains in some aspects.
They are a worthy supplement to a training program. They make holding standard barbells easier, which is great if you are having a hard time in the grip department.